Of the classical problems of physics there is one which might be picked out as simplest to state but slowest to approach solution. "What is the fate of a self-gravitating system of point masses interacting according to Newton's laws?" The frustration felt by theoreticians, who, despite the accumulation of more than 300 years of mathematical knowledge and numerical technology, have not solved the problem, even to understanding the qualitative late evolution of such systems, is compounded by the

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... xistence of observations known since Messier's time of over 100 globular star clusters. These are real and ancient systems, well approximating the idealized problem; astronomical study of their properties should have led us to an understanding of the physical situation. Galactic clusters containing smaller numbers of stars also exist in abundance. However, most of these are much younger systems and they are also subject to a variety of additional complex processes, not important in the large N, globular cluster, systems. Because of this complexity they are less well understood than globulars and will not be covered in this theoretically oriented review.